Refactoring moving members in collections

This refactors the bits of code that add, remove and move a collection's members.  The original version was calling `#members`, which would return ActiveFedora objects, loading everything from Fedora.  This change uses `#member_ids` which makes fewer calls to Fedora and should speed things up a bit. 
Please add `unless ENV['CI']` so the travis build doesn't take forever.